Basschat easy-build lockdown cab project

[Downunderwonder]

Hopefully not going to cause an amp to melt:

 

Edited March 6, 2021 by Downunderwonder

[JohnDaBass]

Hi @stevie & @Phil Starr, 

Internal dimensions of my 'no HF' cab are 375 X 340 X 291 giving a volume of 37Ltrs.  Am I correct that Phil has allowed 7litrs for Speaker, port and HF horn?

If I use a 100mm diameter port 120mm long will I still get close to your planned 50Hz tuning for my cab? 

[Phil Starr]

Hi John, have I missed something here? Stevie has designed the port for this cab using an off the shelf port and is aiming for 58Hz.

You are correct that I allowed 7l for the speaker, ports and braces in the original design but that was for a 12" speaker, the 10 Stevie is using will take up less space but of course the horn will also take up space. 

[stevie]

19 hours ago, Downunderwonder said:

Hopefully not going to cause an amp to melt:

 

Yes, that's about the size of it.

[stevie]

22 hours ago, JohnDaBass said:

Hi @stevie & @Phil Starr, 

Internal dimensions of my 'no HF' cab are 375 X 340 X 291 giving a volume of 37Ltrs.  Am I correct that Phil has allowed 7litrs for Speaker, port and HF horn?

If I use a 100mm diameter port 120mm long will I still get close to your planned 50Hz tuning for my cab? 

The tuning with the standard port is in the upper 50s. The cab does works tuned to that frequency but power handling of the lowest frequencies is compromised. So I'd like to get it down to 50Hz - which means we need to lengthen the port.

There are a couple of ways of doing it. The first is to buy a second port (at £1.27 or what ever not really a problem), cut it to the extra length needed, and use plastic glue (the stuff you use to glue plastic drainpipe) to glue the piece of port to the main one. Support with gaffa tape. The second way is to cut some cardboard (Cornflakes packet, for example) and tape it to the existing port to extend the length.

Once I've tuned the system, I'll advise on the length of port needed.

Edited March 7, 2021 by stevie

[stevie]

Here's the frequency response of the finished system with crossover.

[stevie]

The 70Hz resonance has been bugging me for a few days now, because it shouldn't be there and it's causing a noticeable dip in the frequency response at the same frequency. At first I thought it was an air leak and decided to seal all the battens on the back panel. It didn't make any difference. So I tried another driver and the resonance was still there. That ruled out the driver as the cause of the problem. It had to be the cabinet.

To cut a long story short, I eventually discovered that the impedance blip (or resonance) was caused by vibration of the baffle. The solution - and I'm sure Phil will include this in his drawings - is to fit a brace across the front baffle above the driver.

Edited March 6, 2021 by stevie

[Phil Starr]

It's really interesting, when building the cab I took an instinctive dislike to the baffle arrangement. it left extremely thin pieces of ply between the main driver and the horn and port. I sent Stevie a drawing of the baffle before it was cut and he asked me to move the horn and woofer closer together. Completely the best thing to do from an acoustic point of view as the gap between the speakers needs to be as small as possible to avoid cancellation around the crossover point where they are both working. I actually cut some strips of ply to strengthen the baffle at it's weakest points but left them out as they could always be added later and it would have been almost impossible to remove them cleanly once the glue had set.

It shows the value of the frequency measurements. Listening tests alone wouldn't have identified the problem so specifically and we'd have had a potentially disappointing cabinet. We might have spotted it in testing and my wood working instincts to strengthen weak spots might have come in to play but Stevie's systematic approach nailed the problem fairly quickly. 

You can see the problem here. The pencil lines for my original placement is clear in the picture and they've been moved a couple of cm closer in both directions for the actual cutout, the speaker and the horn both have a lip that overlaps the baffle so they will be almost touching in practice. The metal frame of the speaker will help stiffen the lower part of the baffle and it will be simple to reinforce the baffle from the back. Testing will show what additional bracing might be needed. It will all be in the final drawings.

[nilebodgers]

Just looking at it I would absolutely want a baffle to back panel brace in that design.

[Phil Starr]

2 hours ago, nilebodgers said:

Just looking at it I would absolutely want a baffle to back panel brace in that design.

I know, the cost of lock down. Normally I'd probably just driven across and we could have sorted it on the spot or taken it back for modification. Now we have to rely on couriers we just built a basic box. People like to see a bit of the development too, it helps de mystify the process. 

[stevie]

It's impossible to know whether the resonance is due to the narrowness of the wood between the main driver, horn and port or the thickness of the baffle - we're working with lightweight 12mm ply here. It could well be a combination of both. I did notice that the alternative 10" driver I tried, which has a thicker, cast aluminium chassis, reduced the severity of the resonance, although it didn't get rid of it. I'd guess that an 18mm birch ply baffle wouldn't have the same problem.

When I did more in-depth panel resonance testing on the Basschat MkIII cab (which used 15mm poplar ply),  I noticed that the baffle is by far the worst cuprit for undesireable resonances and that's why I always fit a figure-8 brace nowadays. But this is supposed to be an easy-build design. So I'm going to screw some batten to the back of the baffle and remeasure the impedance curve. If it's successful, I'll show you what the impedance of the system looks like.

[stevie]

There's just enough room to fit some batten.

[Chienmortbb]

Some higher end commercial designs use a thicker, denser ply for the baffle.  Adding say an 18mm birch baffle to a poplar cabinet but again this is a minimal cost build. 

I assume that a cast aluminium frame would help reduce the resonance. Would a ceramic magnet help more than a neo?

[stevie]

And here's a photo of my chocolate block crossover. It should be screwed to a convenient place inside the cabinet. Two screws in the chocolate block and one screw in the inductor - they must be brass or stainless steel screws. Keep all leadout wires as short as possible - the ones on the resistor should be shorter but I left them for clarity. You can tape the whole lot together if you like.

If you want to do a build without any soldering at all, you'll need to source three different sizes of connector because the 10" driver, HF driver and Speakon all use different sizes of crimp.

[stevie]

While I've been doing this, I've been working on another 10" system that uses more expensive components and a much more complex crossover. This is what you can do if money's no object. No smoothing, by the way.

 

Edited March 7, 2021 by stevie

[JohnDaBass]

Does this use the same Pulse 10 speaker? ( Beyma 10WRS 300 perhaps?)

Edited March 7, 2021 by JohnDaBass

[stevie]

No, @JohnDaBasswhen I said more expensive components, I meant a lot more expensive. Their identity will remain a mystery at least for the time being.😊

Edited March 7, 2021 by stevie

[stevie]

This should be the final piece of the puzzle from me. The port needs to be about 170mm long in total. So we have  to add 50mm to the length of our standard 120mm port using one of the techniques mentioned above - unless you can think of another way of doing it. There may well be 4" ports that are longer than the one we're using. If there are, they'll certainly be more expensive.

If you're not bothered about obtaining maximum output from this cab, stick with the tuning that the 120mm length gives you. It'll sound fine. But if there's any chance you might want to put the full 200 watts through it, be sure to add the extra two inches.

For internal damping, I'd line the cabinet with your damping material of choice, keeping it away from the port. Felt is my favourite nowadays, but use whatever you happen to have (or repurpose an old duvet).

I tested the cab with a variety of different recorded sources, including bass guitar (obviously) but also some recordings with voice. The cab is perfectly usable as a PA cab. The colouration between 2 and 3kHz is audible (on bass guitar also) but it doesn't jump out at you. It's a natural sound on voice and doesn't screech on female vocals like a lot of PA cabs do.

Here's the impedance curve without any damping material in the cab. It's a very easy amplifier load. Even @agedhorse would nod in approval (nasty amplifier loads are his pet peeve).

Edited March 7, 2021 by stevie

[stevie]

I forgot to mention -  gluing a piece of batten to the baffle just above the driver completely killed the 70Hz resonance, as can be seen from the impedance curve.

[Chienmortbb]

19 minutes ago, stevie said:

I forgot to mention -  gluing a piece of batten to the baffle just above the driver completely killed the 70Hz resonance, as can be seen from the impedance curve.

What is the scale on the impedance curve @stevie?

[itu]

10 hours ago, stevie said:

Two screws in the chocolate block and one screw in the inductor - they must be brass or stainless steel screws.

The project is really nice, love it!

Even stainless (A4, AISI 316/L) is magnetic, although not as much as plain steel (up to A2, AISI 304 and similar). If you want no issues with the screws, I would recommend brass (or Al, or Ti...).

[stevie]

16 hours ago, Chienmortbb said:

What is the scale on the impedance curve @stevie?

It's on the left, John. Each horizontal bar is 5 ohms. Minimum impedance is around 7 ohms.

[agedhorse]

Impedance looks good 

I will be able to sleep well tonight!

 

[Phil Starr]

Well I've finally broken free of the pressures of work and sent the sketches off to @RichardH who has offered to do the proper drawings for us. One of the lovely thing about BassChat is the generosity of so many people, thanks Richard. I'm going to spend 10 mins uploading pictures of the build, one of which is as blurry as my eyes must have been when I cut the hole for the horn  Once I've got the drawings back and the cab back from Stevie I'll do it all properly as a build thread complete with some written hints, Then it's over to the rest of you to have a go. I hope you feel inspired

[Phil Starr]

So this is the kit of parts, you can just about see my table saw and everything was cut with that. Most people don't have access to machine tools so I'd strongly recommend you get everything cut for you. Machine cutting will give you straight cuts and square edges and make assembly a doddle. The square batten is stuff i cut myself but you should use timber described as PSE (planed square edge) which is usually better quality redwood and the planing guarantees a good 90degree angle 

Time for marking out, mine isn't accurate as i went to felt tip as my pencil wasn't showing well in the photos. This is the top and one of the sides. Ive marked a line set back 30mm for the baffle (front panel where the speaker goes) and for the back panel. Those lines will be used to line the battens up . The top has two other lines as it will sit on top of the sides with the outer edge aligned with the sides.

Edited March 13, 2021 by Phil Starr